search for unknown operator (bpy warning in custom script)
The message:

search for unknown operator "NAME_TYPE_shortname", "NAME_TYPE_shortname"


could come from the user preferences keyboard shortcuts manager. 


Registering an incorrectly spelled operator path will give that message. 
Adding the shortcut in your script before the operator is registered will give that error too.


Maybe this is a typo but your

class updateList(bpy.types.Operator):
    bl_idname = "myplug.update_list"


Would mean that the operator is located at bpy.ops.myplug.update_list(), not bpy.ops.myplugin.* 

There is no inherent reason that operators can't be called within other operators, but you will have to wait for the called operator to complete before the calling operator can end, it doesn't fork off into its own thread.

for instance

import bpy

class login(bpy.types.Operator):
    bl_idname = "myplug.login"
    bl_label = "Login"

    def execute(self,context):
        ...

        # bpy.ops.myplug.update_list()  # should be.  
        bpy.ops.myplugin.update_list()

        return {'FINISHED'}


class updateList(bpy.types.Operator):
    bl_idname = "myplug.update_list"
    bl_label = "UpdateList"

    def execute(self,context):
        print('caramba!')
        return {'FINISHED'}

def register():
    bpy.utils.register_module(__name__)


def unregister():
    bpy.utils.unregister_module(__name__)

if __name__ == '__main__':
    register()
    bpy.ops.myplug.login()


# AttributeError: Calling operator "bpy.ops.myplugin.update_list" error, could not be found


If you are calling a set of operators in sequence, then of course you must  ensure that the operators are registered globally. If i'm not mistaken the bpy.utils.register_module(__name__) will register classes in order of appearance in the file, use that order to your advantage.